Originally cable networks were established to transmit television signals to homes and offices. Cable networks provided advantages over transmission television networks that included providing a clearer signal and a greater selection of channels. These networks were made up of co-axial cables routed in a tree and branch structure to customer sites and were intended simply to provide customers with analog television signals.
More recently, cable networks have been converted to transmit digital signals in a hybrid of fiber optic cable and co-axial cable structures. These converted networks accommodate not only traditional, analog television signals but also digital television signals and digital data signals. Digital television signals provide a crisper, more detailed picture along with enhanced sound. With the capability to transmit digital data signals, cable networks may now be coupled to the Internet thereby providing houses and offices access to the Internet. This Internet access is generally faster than access provided by other technologies.
With digital signals, cable networks utilize a form of digital quadrature amplitude modulation (“QAM”) which provides high speed data transmission in a relatively narrow broadcast band. QAM typically transmits a serial data stream in groupings of multiple bits. QAM uses a combination of amplitude variation and phase angles to encode the bits on a carrier.
One form of QAM, commonly referred to as 64-QAM, transmits six bits at a time. 64-QAM encodes a first set of three bits by amplitude modulating them over eight amplitude levels evenly arrayed about zero. Simultaneously, 64-QAM similarly encodes the second set of three bits. 64-QAM amplitude modulates the first set of bits on a carrier, and simultaneously amplitude modulates the second set of bits on a 90° phase shifted version of the same carrier. 64-QAM then combines the two carriers for transmission. At the receiver, the two signals are separated and the bits decoded. 64-QAM has 64 combinations of amplitude and phase angle; each combination constitutes a symbol.
An alternative form of QAM is 256-QAM where eight bits are encoded into eight bit symbols as two groups of four bits. The technique for 256-QAM is otherwise the same as for 64-QAM. 256-QAM has 256 combinations of amplitude and phase angle; each combination constitutes a symbol.
The two sets of encrypted bits that comprise a QAM signal are referred to as the I component and the Q component, respectively. These are the component signals before the Q component is frequency modulated with at 90° phase angle and combined with the I component. The I component is the encoded first set of three bits, or four bits for 256-QAM, and the Q component is the encoded second set of three bits, or four bits for 256-QAM.
When testing a cable network, it is common to display the I and Q components in a display known as a constellation. FIG. 1 is an illustration of an exemplary constellation for 64-QAM without extraordinary degradation of the QAM signal. The constellation for 64-QAM has 64 groupings of points in the 8×8 matrix; for 256-QAM, the constellation has 256 groupings of points in a 16×16 matrix. The groupings of points are frequently referred to as “cells.”
To generate a constellation, a test device plots the location of the received symbols on a coordinate axes. When plotting the received symbols, the horizontal axis represents the I component of the signal, and the vertical axis represents the Q component of the signal. Once plotted, the symbol becomes a datum point of the constellation.
The constellation is divided into 64 square cells, or 256 square cells in 256-QAM. There is a cell for each symbol of the 64-QAM signal. In an ideal network, the received symbol would plot exactly in the middle of its cell in the constellation. FIG. 1 shows the data points distributed about the ideal center point of each cell. This dispersion indicates noise or impairments in the network.
As FIG. 1 shows, cable networks are not ideal. From an analysis of the constellation, however, it is possible to glean information about impairments in a cable network. Once identified, these impairments may be corrected or lessened. Four common defects that occur in cable networks are phase noise, compression, coherent interference, and non-coherent interference.
FIG. 2 shows a constellation for a network where phase noise is present. Phase noise consists of deviations from the ideal phase angles of 90°, which QAM uses when combining the I and Q components. Phase noise distorts the constellation such that it appears to be twisted about the origin. FIG. 2 shows an extreme case of phase noise where its presence is readily apparent.
Another defect in the signal on a network is compression. Compression occurs when one or more amplifiers in the network do not accurately reproduce their input signal. FIG. 3 shows a constellation where the signal is extremely compressed. Compression is evident because the cells at the outer corners of the constellation appear to be pushed toward the origin. The constellation takes on the appearance of being bent around a sphere.
Coherent interference is yet another possible defect in the network. A carrier signal interfering with the digitally modulated QAM signal causes coherent interference. FIG. 4 shows the constellation where the signal has experienced extreme coherent interference. The degraded QAM signal appears in the constellation as symmetrical deviations from the ideal center point. As FIG. 4 shows, for any one cell, the points are arrayed symmetrically around a void at the center of the cell.
The final possible defect in the network that is addressed here is non-coherent interference; one common form of non-coherent interference is laser clipping. Non-coherent interference occurs when a driver in the network attempts to exceed its output range. That is the total signal power exceeds the ability of the driver to replicate the signal. Non-coherent interference appears in the constellation as stray points away from the clusters within the cells. FIG. 5 shows a constellation where non-coherent interference is present in the signal.
Until now, technicians have had to diagnose phase noise, compression, coherent interference, or non-coherent interference in a network by a manual observation of the constellation. As FIGS. 1-5 show, when extreme cases of these defects are present, a technician can readily identify their presence. The situation is much more difficult when the defects are not extreme. In less extraordinary situations, a technician may not be able to identify the presence of these defects in the network. This difficulty is compounded when more than one of the defects is present. Multiple defects can interact thereby masking their presence to a technician who simply observes a constellation. It becomes extremely difficult for a technician to troubleshoot accurately and efficiently a network simply by observing its constellation.
There is a compelling need for systems and methods capable of troubleshooting accurately and automatically cable networks.